In a typical marine seismic gathering system, it is customary that a vessel be equipped with both an acoustical energy source, usually on a submerged carrier towed by the vessel with certain control apparatus therefor being located on the vessel itself, and an acoustical detector array, usually in the form of a complex cable also towed by the vessel. Such a detector cable is typically towed at a shallow depth behind the vessel and is best characterized as a streamer or an extended cable including a plurality of seismic detectors or hydrophones. It is also usual for such detectors to be spaced along the streamer in multiple arrays, rather than singly. The towed streamer of the prior art is ideally neutrally buoyant and seeks a uniform depth beneath the surface of the water, usually in the vicinity of from one to three meters. The primary reason that the streamer is towed below the water surface is to avoid, insofar as possible, the effects of surface wave action or turbulence.
The returns detected by the hydrophone arrays are a result of the acoustic impulses from the source being reflected from the various subsurface seismic interfaces. One such interface is the interface between the water and the land, or in other words, the lake or ocean bottom. Other interfaces occur wherever there is a lithological variation or change. Knowledge of such interfaces or reflecting surfaces is extremely valuable in evaluating for the presence of hydrocarbon deposits and the like.
The gathered acoustic return data using such a streamer of hydrophone arrays is subjected to several natural phenomena which interfere with a clear interpretation of the data collected unless avoided or minimized and/or corrected for. One of these phenomena is surface noise. It is well-known that a hydrophone located at or near the water surface will pick up surface wave motion. Therefore, it has been found convenient to locate the hydrophone detectors below the water surface, typically on the order of one to three meters (although such below-surface location introduces ghost returns, which are discussed below).
Another recognized phenomena that must be considered before the collected data is clearly interpretable is the phenoemna known as correcting to a common depth point (CDP) file. Data may ideally be gathered to a common depth point; however, as will be explained, it is not normally practical to do that, particularly in a marine configuration setting. But, for an understanding of the concept, consider a horizontal reflecting interface with a point thereon as the "CDP". Along a parallel "datum" line above the interface, and to one side of a normal drawn to the CDP, are evenly spaced detectors. (Actually, there is normally a detector array, but for discussion herein "detector" is used to signify an associated arrayed group of individual detectors.) Along the datum line and to the other side of the normal drawn to the CDP, are equally evenly spaced sources. A first data trace would be the result of an impulse from the closest source being reflected off the interface and received at the closest detector. A second data trace would be the result of an impulse from the next closest source being reflected off the interface and received at the next closest detector. Similarly, data traces developed from successive sources to successive detectors, each resulting from a reflection off the interface at the CDP, would develop a "common depth point file".
However, there is normally only one source in a typical marine seismic system, which source is towed at a predetermined rate. Assuming that the detectors were stationary and evenly spaced, when the source was at a position corresponding to the first source in the above example, then the second, and so forth, an ideal CDP file could be developed. In the normal system, however, the detectors are not stationary, but are towed in conjunction or at the same rate as the source. Therefore, it may be seen that a two-trace, or "two-fold" common depth point file is developed when the source is impulsed at an initial position and then impulsed again when it and the detector cable have been towed together one-half of the detector spacing distance, the first impulse being detected by the first detector and the second impulse being detected by the second detector. The process can then be repeated for as many detectors as there are on the cable for a full-fold CDP file.
Of course, data is not actually collected in the field in the manner just described. In actual practice, a source impulse is detected at all of the detectors, but not from a common depth point. Then at a second location of the source, which normally would be at a distance from the place where the source was first impulsed, the source is again impulsed and detected at each of the detectors, again following reflection from different depth points. From the individual field recordings, data associated with a common depth point is selected and is built up in what is truly a common "CPD file". Hence, interpretation is not from the field recordings but from the CDP files.
Because the travel time for an impulse from the source to the reflecting interface to the detector is longer for the second detected trace than for the first detected trace in a CDP file, and for the third detected trace than for the second detected trace, and so forth, a correction is necessary for the subsequent data traces or events to position them in time with the first data trace or event. Such correction is referred to as the normal moveout (NMO) correction. Factors involved in making such correction, which is different for each detector event resulting from a successively spaced detector, are well-known in the art and are explained, for example, in Geophysics, a publication of the Society of Exploration Geophysicists, Vol. 27, No. 6, published in 1962 at page 927, in an article entitled "Common Reflection Point Horizontal Data Stacking Techniques", W. H. Mayne, which is incorporated herein by reference for all purposes.
Distortion caused by cable droop is usually just tolerated. The buoyancy of a cable can be modified to achieve an adjusted location that is more parallel to the surface when there is an appreciable deviation therefrom. It is also possible to correct droop-distorted data by determining the amount of droop by a measurement and then correcting the data collected to the surface "datum" line, such as for correcting for uneven land surface swells in a land seismic system. This correction is usually done even when the cable is approximately parallel to the water surface anyway.
Unwanted noise, other than mere static or random noise, is a frustrating phenomenon that is also usually just tolerated. Such noise can arise out of the vertical plane or profile of the cable and may be the result of a source not related to the seismic source employed in the system or it may be the result of a reflection other than an lithological interface barrier from below. For example, a noise progressing underwater at a sideways angle to the cable constitutes such noise.
Perhaps the most distributing and hardest to correct of all external effects however, has been that data effect introduced by ghost reflections. A signal from the source progresses downward through the water until it is reflected upward by the interface at the bottom of the water to be received by the hydrophone. In addition, however, there is a reflection that continues to the surface where it is reflected downward by the water-to-air interface to be received at the hydrophone at a slightly later time than the direct or primary reflection. This reflection is referred to as the ghost reflection. The combined effect of the primary reflection and the ghost reflection is a distorted wave compared to the wave appearance of the source impulse. For example, assuming a source impulse having a broad frequency spectrum, the relative amplitude in the frequency domain being approximately centered about a mid frequency and gently rolling off therefrom over about three octaves, the arrival of the primary and its ghost reflections at a detector will produce a multiple humped-shaped response in the frequency domain having a notch or notches between each hump. The interrelated effect of the ghost reflection with the primary reflection can be analyzed to determine that at some frequencies within the spectrum there is interference cancellation and at other frequencies there is interference augmentation of reinforcement, resulting in amplitude distortion over the entire spectrum range. For each interface, there is a primary reflection and a ghost reflection. The distortion in the shape of the frequency domain response depends on the distance that the ghost reflection is from the primary reflection. The further the two are apart, the larger are the number of notches.
Since the results of the interaction of a ghost reflection on its primary reflection is subject to analysis, it is common to design an inverse electronic filter to correct for the amplitude distortion which results. In a very real sense, when compared to an ideal undistorted response, the actual reflected response can be viewed as having been subjected to an unwanted analog filter caused by the interface reflections and the mediums through which the reflections travel. Therefore, the purpose of inverse electronic filters employed in the prior art systems is to restore the reflected event response to appear as the source pulse, which it may be remembered in the above example, was shaped to have a smooth single hump in the frequency domain, its center frequency amplitude gently rolling off on either side thereof for about three octaves.
It is apparent that such compensating filter amplifies frequencies close to the notch greatly in order to restore the lost resolution. In doing so, it is also readily apparent that such inverse filter introduces noise and thereby introduces a signal-to-noise loss. The presence of an inverse filter also has the effect of reducing penetration of the effective source transmission and reflection reception since noise amplification is inherent and, hence, unavoidable.
For combination primary and ghost responses developed at detectors progressively further from the source than the near detector, as mentioned above, the Fourier transform response caused by the ghosting phenomenon creates so-called "trace depth notches", at slightly different locations from the notch of the response at the first detector. It should be noted, therefore, that the ghosting phenomenon introduces a phase as well as an amplitude distortion. Hence, to correct for both amplitude and phase distortion of these trace depth notches in these responses, it has been a practice in the prior art, at the appropriate phase positions involved (in other words, at the slightly different notch locations for the responses associated with each detector), to insert inverse filtering during the data processing stage. Such processing introduces compensating amplification at the notch locations and compensating attenuation for the sharp sides of the response on either side of the notches.
Therefore, it is a feature of the present invention to provide an improved high resolution marine seismic stratigraphic system that avoids, in the data handling portion of the system, the use of an inverse filter.
It is another feature of the present invention to provide an improved marine seismic stratigraphic system which, in gathering data and in its complementary treatment increases penetration with the same strength source as used in prior art systems by operating in such a manner to avoid amplifying noise. Hence, it is possible to achieve operation at a higher signal-to-noise ratio than that which was inherent in prior art systems.
It is still another feature of the present invention to provide an improved focused or directed marine seismic stratigraphic system, which as an overall system of data collection and processing, attenuates noise directed at the arrays of detectors located along a cable or streamer other than from the vertical direction by 6 db or more.